To satisfy demands for wireless data traffic having increased since commercialization of 4th-Generation (4G) communication systems, efforts have been made to develop improved 5th-Generation (5G) communication systems or pre-5G communication systems. For this reason, the 5G communication system or the pre-5G communication system is also called a beyond-4G-network communication system or a post-long term evolution (LTE) system.
To achieve a high data rate, implementation of the 5G communication system in an ultra-high frequency (mmWave) band (e.g., a 60 GHz band) is under consideration. In the 5G communication system, beamforming, massive multi-input multi-output (MIMO), full dimensional MIMO (FD-MIMO), an array antenna, analog beamforming, and large-scale antenna technologies have been discussed to alleviate a propagation path loss and to increase a propagation distance in the ultra-high frequency band.
For system network improvement, in the 5G communication system, techniques such as an evolved small cell, an advanced small cell, a cloud radio access network (RAN), an ultra-dense network, a device to device (D2D) communication, a wireless backhaul, a moving network, cooperative communication, coordinated multi-points (CoMPs), and interference cancellation have been developed.
In the 5G system, advanced coding modulation (ACM) schemes including hybrid frequency-shift keying (FSK) and quadrature amplitude modulation (QAM) modulation (FQAM) and sliding window superposition coding (SWSC), and advanced access schemes including filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) have been developed.
In a wireless communication system, to operate a system with low complexity (for example, to operate adaptive modulation and encoding, to generate a soft decision decoding metric, etc.), a Gaussian assumption for an interference signal is used. To make characteristics of an interference signals as close as possible to a Gaussian distribution, a quadrature amplitude modulation (QAM)-based modulation scheme will be mainly used. In addition, for a user that may not satisfy target error performance even if a minimum channel encoding rate and a minimum modulation order are applied, a QAM symbol is repeatedly transmitted to achieve target performance.
However, it has recently proved that if a statistical distribution of an additive noise follows a Gaussian distribution in a wireless communication system where a strong interference signal exists, this case is worst in terms of a channel capacity. Thus, if a statistical distribution of interference signals having additive noise characteristics is designed to follow a non-Gaussian distribution, a higher system throughput than that of a conventional system is apparently obtained. A modulation scheme proposed for this reason is a frequency quadrature amplitude modulation (FQAM) scheme.
FIGS. 1 and 2 show characteristics of a FQAM scheme in a conventional wireless communication system.
Referring to FIG. 1, a FQAM scheme (c) is a hybrid modulation scheme that is a combination of a QAM scheme (a) and a frequency shift keying (FSK) (b), in which some of multiple subcarriers of a symbol are activated, such that a statistical distribution of an interference signal has characteristics of a non-Gaussian distribution. This is similar with a conventional FSK scheme. However, the FQAM scheme transmits a QAM symbol through activated subcarriers, thereby largely improving spectrum efficiency when compared to the FSK scheme. As shown in FIG. 2, if the FQAM scheme is applied to cell edge users having strong interference signals, a non-Gaussian interference channel is formed, thereby largely improving a conventional system throughput when compared to a system that forms a Gaussian interference channel by repeatedly transmitting a QAM symbol.
However, to obtain remarkable performance improvement when compared to a conventional technique by applying a modulation scheme such as the FQAM scheme, application of non-binary encoding/decoding is essential. This is because the FQAM scheme is more suitable for a non-binary code than a binary code due to characteristics of a distance between candidate transmission signals. However, a non-binary encoding/decoding technique, which is essential for application of the FQAM scheme, is highly complex, causing a problem in implementation. A modulation scheme proposed to solve the problem in implementation is a nulling quadrature amplitude modulation (NQAM) scheme. The NQAM scheme is a scheme that increases a transmission power of a symbol modulated using a QAM scheme, let some subcarriers be empty, and applies a subcarrier permutation rule specific to each cell. The NQAM scheme forms a non-Gaussian interference channel which is similar with for the FQAM scheme, unlike conventional repeated transmission of a QAM symbol. The NQAM scheme is a binary encoding/decoding technique unlike conventional FQAM, and may largely improve performance in comparison to an existing QAM scheme.
Meanwhile, in a cellular wireless communication system, there are a downlink through which a base station delivers information to a terminal and an uplink through which the terminal delivers information to the base station. The uplink through which the terminal delivers information to the base station needs to minimize battery consumption of the terminal, such that a peak to average power ratio (PAPR) problem is regarded as being important. Thus, in a standard related to a cellular wireless communication system, a single carrier frequency division multiple access (SC-FDMA) scheme is applied to the uplink to reduce the PAPR.
The FQAM scheme and the NQAM scheme, when applied to an orthogonal frequency-division multiple access (OFDMA), have a PAPR that is similar with the QAM scheme, but have a much higher PAPR than the QAM scheme when applied to the SC-FDMA scheme. This is because the FQAM scheme and the NQAM scheme activate only some of multiple subcarriers of a symbol. If the FQAM scheme or the NQAM scheme is applied to the SC-FDMA system in place of the QAM scheme, a non-Gaussian interference channel is formed, but a transmission power needs to be reduced in comparison to the QAM scheme due to the PAPR problem, largely degrading overall network throughput improvement.
A modulation scheme proposed to solve this problem is a sequence quadrature amplitude modulation (SQAM) scheme. The SQAM scheme removes empty subcarriers of the FQAM scheme by applying sequence modulation instead of the FSK scheme of the FQAM scheme, and adds a correlator for a received signal to a receiver, causing an interference signal observed in the receiver to have a non-Gaussian distribution that is similar with for the FQAM scheme. Thus, by forming a non-Gaussian interference channel while maintaining PAPR characteristics of the SQAM scheme similarly with the QAM scheme, performance has been largely improved in comparison to the existing QAM scheme. However, to improve performance when compared to the existing QAM scheme by applying the SQAM scheme, application of a non-binary encoding/decoding technique is essentially needed. For this reason, in a system using the SQAM scheme, a problem in implementation is very likely to occur due to the complexity of the non-binary encoding/decoding. Therefore, there is a need for a modulation scheme capable of forming a non-Gaussian interference channel without causing the PAPR problem, and improving performance with a binary encoding/decoding technique in comparison to a conventional QAM scheme, in an SC-FDMA system.